Shamima Begum’s return to the UK has been put on hold after the government was given permission to take the case to the Supreme Court.
Begum, now 20, was one of three east London schoolgirls who travelled to Syria to join the Islamic State (IS) terror group in 2015.
She lived under IS rule for more than three years before she was found, nine months pregnant, in a Syrian refugee camp in February last year.
Then-home secretary Sajid Javid revoked her British citizenship on national security grounds later that month.
Earlier this month, Court of Appeal judges ruled that Ms Begum should be allowed to return to the UK to challenge the decision, saying it was “the only way in which she can have a fair and effective appeal”.
The judges found that “the national security concerns about her could be addressed and managed if she returns to the United Kingdom”.
But at a remote hearing on Friday, the government argued that the Court of Appeal wrongly concluded Ms Begum should be granted leave to enter the UK.
Lawyers for the Home Office said there were “significant national security concerns” about Ms Begum’s potential return.
Sir James Eadie QC said: “The public will be less well protected if Ms Begum returns than it would be if she does not return.”
He argued that the case concerned “an issue of real pressing public importance”, namely what the courts should do when someone “cannot have a fair and effective hearing in a deprivation appeal, but they cannot do so because… of going abroad and aligning with terrorist groups”.
Lady Justice King said the case raised “points of law of general public importance” which should be considered by the UK’s highest court.
She said that “clearly there must be a stay” on Ms Begum’s potential return to the UK “until further order by the Supreme Court”.
The judge also granted permission for Ms Begum’s lawyers to challenge the decision that the absence of a fair and effective appeal did not mean her British citizenship should be restored, subject to the Supreme Court accepting that part of the case.
Ms Begum was one of three schoolgirls from Bethnal Green Academy who left their homes and families to join IS in February 2015.
In an interview with Sky News last year, she claimed she was “just a housewife” during her four years in IS’ self-declared caliphate, where she married a young Dutch fighter called Yago Riedijk three weeks after arriving.
She said she left Raqqa in January 2017 with her husband but her children, a one-year-old girl and a three-month-old boy, had both since died.
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Her third child, called Jarrah, died shortly after he was born last year.
Ms Begum travelled to Syria to join IS with fellow Bethnal Green Academy pupils Kadiza Sultana – who is believed to have died – and Amira Abase, whose fate remains unknown.